“Many species of microbes associated with the mouth are also present in the gut and may flourish there,” says co-corresponding and lead author Jihyun F. Kim of Yonsei University in Seoul. “These observations made us wonder whether oral microbes reach and persist in the gastrointestinal tract differently in people with cancer.”

Tracking microbes from mouth to gut

To conduct the study, the researchers recruited 507 volunteers to donate both oral and fecal samples, using a highly standardized collection process. The cohort included 129 healthy individuals; 215 people with metabolic disorders such as metabolic syndrome, hypertension, hyperlipidemia and type 2 diabetes; 77 people with gastric cancer; and 86 people with colorectal cancer. For the volunteers with cancer, samples were collected before the start of any treatment.